The goal of this proposal is to determine the role of changes in intracellular calcium activity, (Ca)i and calcium influx in mediating neurotransmitter regulation of bursting. Bursting occurs when neurons fire clusters of action potentials usually at higher than normal frequencies. We are studying bursting because it is a central and fundamental phenomenon of epileptogenic neurons and therefore understanding bursting is crucial to understanding epilepsy. We are focusing on neurotransmitter regulation of bursting because understanding such regulation may open new avenues to the pharmacologic control of bursting with clear implication for epilepsy treatment. We are using the invertebrate neuron R15 as a model because a) we can monitor (Ca)i and calcium influx in this cell; b) we can elicit controllable neurotransmitter induced modulation of bursting in this cell; and c) bursting in R15 has many features in common with bursting in epileptogenic hippocampal pyramidal neurons. Our specific aims are to 1) determine if the blockade of bursting by bath application of dopamine is associated with an increase in (Ca) i or altered calcium influx; 2) determine if any observed effects of dopamine on (Ca)i or calcium influx can be blocked by the specific dopamine antagonists dihydroergocryptine or lysergic acid diethylamide which prevent the blockade of bursting by dopamine in R15; 3) determine whether alterations in (Ca)i or calcium influx might occur during endodogenous synaptic intensification or inactivation of bursting in R15. Voltage clamp techniques will be used to activate membrane conductances and measure current. Arsenazo III spectrophotometry will be used to monitor changes in (Ca) i in the soma of R15. These are neurophysiological techniques that we have demonstrated our ability to use (manuscripts appended). Current literature supports our hypothesis that calcium may act as a second messenger to mediate neurotransmitter modulation of bursting. Exploring this possible mechanism may add significantly to our knowledge of bursting and epilepsy.